The present invention relates to the discharging of static electricity, and particularly to a device and method for removing static charge from a surface ionization of air.
Static electricity is defined as surface storage of electric charge. This surface charge is caused by the transfer of electrons when two similar or dissimilar surfaces contact. The charge also creates a voltage field which attracts or repels other objects which are proximate to the field. (This attraction or repulsion can create problems, as will be discussed further hereinafter.) This voltage pressure or potential induces out from the surface in all directions when the charged object is in space. It is the induced voltage pressure which pushes the static charge to ionize.
Ways of controlling static charge are known. Induction static eliminators take advantage of the voltage field around a statically charged surface by inducing ionization of the air at or near conductive points (or ends) of small cross-sectional area within the voltage field. The voltage pressure or potential is increased around the conductive points, inducing ionization of the air; the ionized air and the conductive points provide a path to ground for the charge. Known induction static eliminators typically use a fixed row of grounded bundles or brushes of conductive threads or fine wires, or grounded strips, e.g. of copper, held perpendicularly to a passing surface to touch or nearly touch the charged surface to ionize the air within the voltage field. The small cross-sectional area at the tip of each thread or fine wire or end point of each strip increases the voltage pressure or potential at the tip, end, or point (hereinafter "point"), inducing ionization of the air surrounding the point.
Alternatively, the grounded conductive material may be in the form of "tinsel" (a strand of twisted-together conductive strips with protruding strip ends) or a conductive sheet with one edge cut into jagged points. The small cross-sectional surface area of these strip ends or points and the proximity of these "points" to the charged surface enables ionization of the air within the voltage field and conduction of the charge to ground in the same manner as that described above.
Electrically powered static eliminators are similar to inductive eliminators, in that similar ionizing "points" of small cross-sectional surface area are arrayed perpendicularly to the charged surface. Alternating positive and negative voltages typically are applied to the ionizing "points" to ionize the air around the points to neutralize nearby charges.
Nuclear static eliminators also ionize surrounding air to neutralize static charge. Strips of radioactive material, typically in the form of a foil, provide ionizing "points" which emit alpha particles, producing positive and negative ions which, in turn, exchange electrons with the charged surface molecules to neutralize them.
The typical method for controlling static on conductors, e.g., metal objects or people, is to ground them. For example, a charge buildup may be prevented on a human operator by providing a path to ground for the charge by such means as grounded conductive mats, conductive wrist straps, and conductive shoe straps. However, only objects that will conduct electrical energy or charge can be grounded.
The major problem in static control on insulators, e.g., plastics, synthetics, or paper, is that by definition insulators cannot be grounded. Further, when an insulative material contacts a grounded conductive surface, the insulator cannot give up its surface charge. Also, there will be a transfer of electrons taking place due to such contact, which can further charge the surface of the insulative material. The insulative surface, having a greater affinity for electrons, will oftentimes build up a negative charge, while the opposite polarity generated on the conductive surface will instantaneously be conducted to ground. Thus, even if machine surfaces are made from a metal or other conductor(s) and are grounded, they cannot eliminate static charge buildup on non-conductive objects or materials coming in contact with them. Further, a static charge can be generated on surfaces of such non-conductors by their contact with grounded conductors.
A major limitation of the prior art static eliminators described above is that they are only effective when the field of the charged object is undisturbed and in space (i.e., not in contact with other objects). When a charged object is in contact with or in close proximity to another object or surface, the field is disturbed and induces toward the other object or surface. For example, when a flat material such as a sheet of plastic is charged and placed in contact with another flat surface, the charge on the plastic sheet induces toward the other surface, causing the plastic sheet to cling to the flat surface. Concurrently, the voltage field on the opposite (non-contacting) side of the plastic sheet is not available for induction to nearby static eliminator ends or points or for charge neutralization by positive and/or negative ions generated by an active alpha or electric ionizer.
This phenomenon can be explained by the formula for static charge, EQU C=V/Q
where C represents the static charge, V represents the voltage, and Q represents the capacitance of a statically charged material. When a charged object is in space, Q=1. Thus all of the voltage pressure is available for static removal by induction or active ionization. As the capacitance, C, increases due to proximity of the statically charged object to another object, less voltage pressure (V) is available for induction or active ionization.
Even more problematical is the fact that an insulative material in motion can contact another surface causing triboelectric generation of static charge and the resulting cling without ever separating from the surface. Static generation is most commonly observed when similar and dissimilar materials contact and separate. However, the static generation occurs as soon as one material touches the other. As the molecules of one material closely approach those of another material, there is a transfer of electrons, generating a static charge. Whenever there is high capacitance and insufficient voltage pressure to induce or actively ionize, contact between objects will generate static charge and the resultant static problems, i.e., cling, drag, misalignment, electrostatic discharge (ESD), etc.
One example of the deleterious effects these problems can have is in the die cutting of thin, light, insulative materials such as foam or paperboard pieces or packaging materials. As the die compresses and cuts the material, there is contact between the die surfaces and the small cut pieces resulting in static generation in the cut pieces and clinging of the cut pieces to the die surfaces. Because the transfer of electrons and the cling occur almost instantaneously and while the surfaces are in intimate contact, conventional static eliminators cannot neutralize the charge by induction or active ionization.
In another example, a fine filament, e.g., a thread, fiber, or yarn, is passed through a conduit, e.g., a tube or pipe, which supports it through space. An example of such apparatus is an air blown piping system. Similar apparatus may be used to transport powders or particulate materials. There is contact between the filament, powder, etc. and the walls of the conduit, generating a static charge. The resulting cling or drag can cause severe handling problems. Even the use of conductive plastic or metal conduit does not solve the problem; in some cases the problem is even more severe due to triboelectric generation of static charge on the insulative filament as the dissimilar materials of the filament and conduit contact and separate. Conventional static eliminators are not effective in this application not only because of the capacitance of the charged filament within the conduit but also because of space restrictions within the conduit itself.
Yet another example involves the transport of a light, flat, insulative material, e.g., paper, plastic, fabric, etc., across another flat surface without continuous support, e.g., a flat envelope contacting the side surfaces of a machine, a sheet of paper sliding down a feed board of a printing or copy machine, a fabric sliding across a flat surface of a cutting machine, or a thin sheet of plastic film moving across the flat surfaces of a film processing machine. In each case the material, by contact with the machine surface, can develop a static charge which results in handling or ESD problems. While the material is in contact with the machine surface, it has a higher capacitance and a reduced voltage pressure; thus the static charge cannot be effectively removed by conventional static eliminators.
There are many similar applications where a moving material contacts a similar or dissimilar material resulting in static charge, and where the resulting problems of cling, repulsion, or ESD cannot be addressed by known means.
It would be desirable to have a means of eliminating the static charge on such materials using the ionizing mechanisms described above, but with the ionizing means being in a form integral with the surfaces over which the sheet material must pass. For example, it would be desirable to have a static eliminator which can be installed to cover or be incorporated into a surface directly under the moving sheets as they pass through the machine and/or in closed or restricted areas of the machine. Prior art static eliminators are too bulky to be useful in such ways.
In another context, conductive fabrics have been developed for static control either by weaving conductive threads together to form a fabric or by weaving the threads into a fabric with other non-conductive threads. Alternatively, long conductive threads have been pressed to form a felted mat, alone or with non-conductive threads. Such fabrics provide the small cross-sectional area fiber ionizing "points" required to effectively control static on moving sheet materials only at the cut edges of the fabrics. Because insulators, by definition, cannot be grounded, these types of conductive fabric do not ionize across their flat surfaces. Although there might be some reduction of high voltage to a conductor or conductive fabric surface, these fabrics are not efficient at inducing static charges to ionize.
The low profile ionizing surface described herein was developed to address the need for a flat, non-bulky, low profile sheet-, strip-, ribbon-, or tape-form static eliminator capable of neutralizing static charge on an insulative material on or near its flat, low profile surface.